SITE OF ACTION OF HYPOXIA AND INHALED NO IN THE PULMONARY CIRCULATION OF NEWBORN PIGS DETERMINED BY MICROFOCAL ANGIOGRAPHY. • 1795

Exposure to alveolar hypoxia provokes pulmonary vasoconstriction and administration of NO via inhalation provokes pulmonary vasodilatation. We hypothesized that hypoxia and inhaled NO (iNO) delivered in the alveolar space will cause direct and opposite arterial diameter changes in the arteries responding to alveolar gas. Lungs of 9 newborn pigs were isolated and perfused with autologous blood and 5% Dextran at a constant flow under zone III conditions. Microfocal angiography was performed during ventilation with a normoxic gas mixture (PO₂ 100mmHg, PCO₂ 40mmHg), with the normoxic gas mixture and iNO (35 ppm) and with a hypoxic gas mixture(PO₂ 40mmHg, PCO₂ 40mmHg). The arterial pressure decreased significantly with iNO and increased significantly with hypoxia. The graph shows the changes in vessel diameter as the result of iNO and hypoxia compared to normoxia, and both hypoxia and iNO elicited responses in vessels < 900μm. Furthermore, the smallest vessels had the greatest response to the alveolar gas mixtures. We conclude that the pulmonary arterial response to changes in the alveolar gas composition is confined to arteries less than 900μm in diameter. *p < 0.05. Funded by the AHA and Dept. of VA Figure